This invention relates to radioactive logging and more particularly to a method for simulating the radioactive content of a subsurface formation for use in the testing and calibration of uranium assay logging tools employing prompt fission neutron detection, delayed fission neutron detection and gamma radiation detection.
A natural earth formation may be characterized by its state of radioactivity and more particularly with regard to its uranium content on the basis of prompt and delayed neutrons resulting from induced neutron fission of uranium. When a formation containing a uranium ore is irradiated with neutrons, a certain proportion of the uranium nuclei react to neutron bombardment by breaking into smaller nuclear fractions which normally are referred to as fission products. The fission of uranium is attended by the emission of prompt neutrons immediately upon occurrence of the fission reaction and also by the emission of delayed neutrons subsequent to the fission process. The delayed neutrons are emitted by the fission products for an appreciable length of time following the fission reaction.
U.S. Pat. No. 4,180,730 to Givens et al discloses a prompt fission uranium assaying operation which is carried out by locating in a borehole adjacent a formation of interest a logging tool which includes a source of fast neutrons, epithermal neutrons and thermal neutron detectors. The formation is irradiated with repetitive bursts of fast neutrons and subsequent to each burst and, after dissipation of the original source neutrons, prompt fission neutrons resulting from the thermal neutron fission of uranium-235 are detected. These prompt fission neutrons are produced and detected within a few tens of microseconds after each neutron burst from the source of fast neutrons.
U.S. Pat. No. 3,845,299 to Givens discloses a delayed fission uranium assaying operation carried out in similar manner to that described in the aforementioned patent to Givens excepting that subsequent to each burst of fast neutrons the delayed fission neutrons resulting from the neutron fission of uranium are detected. Those delayed fission neutrons are produced and detected in the order of a few milliseconds after each neutron burst from the source of fast neutrons.
As further noted in U.S. Pat. No. 3,845,299, it may be preferable to run a natural gamma radiation assay to identify those formations suspected of containing uranium and for which a neutron fission assaying operation should be carried out.